Intestinal adaptation is a critical, compensatory response to massive small bowel resection (SBR) and characterized by increased enterocyte turnover as gauged by elevated rates of both proliferation and apoptosis. The significance of apoptosis to the magnitude of adaptation was revealed during the previous funding cycle of this grant as amplified adaptation was observed when apoptosis was actively inhibited. While the mechanism(s) for elevated apoptosis after SBR is presently unknown, we have established that this response is regulated by epidermal growth factor receptor (EGFR) signaling and requires expression of the proapototic Bcl-2 family member Bax and the transcription factor signal transducer and activator of transcription (STAT)-1. As an extension of these key observations, we propose the global hypothesis that EGFR signaling modulates the expression and activity of Bax to regulate resection-induced apoptosis. To test this hypothesis our aims are: 1) Determine the role for STAT-1 in the regulation of Bax expression during intestinal adaptation. STAT-1 expression and activity will be determined in the ileum after SBR as well as in cell culture following induction of apoptosis. The effect of STAT-1 deficiency on Bax expression and apoptosis will be measured and putative STAT-1 binding sites on the Bax promoter will be investigated. 2) Determine the role for p38alpha mitogen-activated protein kinase (MARK) as a modulator of Bax activity during resection-induced adaptation. A temporal and spatial profile of p38 expression/activity and Bax activation will be recorded after SBR in mice and complementary in vitro models of apoptosis. The effect of conditional, intestine-specific deletion of p38 expression on Bax activity and apoptosis will be determined after SBR. 3) Determine the mechanism for EGFR regulation of Bax expression and activity. The effect of enhanced or disrupted EGFR signaling on STAT-1 and p38 activation and expression will be recorded. The effect of attenuated STAT-1 or p38 expression in the context of EGFR inhibition on apoptosis and Bax activity and expression will be determined. The proposed studies in this application will identify the most relevant signaling pathway to direct apoptosis after massive SBR. A thorough understanding of the precise mechanism for induction of apoptosis is fundamental for bench-to-bedside translation of therapeutic targets intended to maximally stimulate regrowth of the intestinal mucosa in response to massive intestinal loss.